The present invention relates generally to a nonvolatile memory using a resistance material, a memory system incorporating the nonvolatile memory, and a method of driving the nonvolatile memory.
Nonvolatile memories using resistance materials include phase-change random access memories (PRAMs), resistive RAMs (RRAMs), and magnetic RAMs (MRAMs). While dynamic RAMs (DRAMs) or flash memories store data in relation to electrical charge, nonvolatile memories having resistance materials store data in relation to the states of a phase-change material such as chalcogenide alloy in the case of PRAMs, the resistance of a variable resistance material in the case of RRAMs, or the resistance a magnetic tunnel junction (MTJ) thin film of a ferromagnetic material in the case of MRAMs.
In particular, the phase-change material of a PRAM becomes crystalline or amorphous as it is cooled after being heated. The phase-change material has low resistance in the crystalline state and high resistance in the amorphous state. The crystalline state has been defined as indicating set data or a data value of 0, and the amorphous state as reset data or a data value of 1.
A PRAM performs a write operation by providing a set pulse or a reset pulse to the phase-change material in order to affect a joule heating. In order to write set data to the conventional PRAM memory cell, its phase-change material is heated to a temperature higher than its melting point in response to the reset pulse and then is quickly cooled. This heating and cooling sequence causes the phase-change material to assume an amorphous state. In order to write reset data, the phase-change material is heated to a temperature which is higher than a crystallization temperature and lower than the melting point, this temperature for the phase-change material is maintained for a predetermined period of time and then cooled. This heating and cooling sequence causes the phase-change material to assume a crystalline state. Here, the time required to write set data is approximately five times longer than the time required to write reset data. For example, the time required to write set data in certain conventional examples is approximately 600 ns, while the time required to write reset data is approximately 120 ns.